The invention is in a field which is now commonly designated as digital subtraction angiography. The invention disclosed herein pertains to methods and apparatus for performing hybrid subtraction of x-ray images which results in a high ratio of the signals representative of the subtracted images to noise. More specifically, the invention pertains to maximizing image representative signals relative to noise by using matched filter methods and apparatus.
A matched filter for x-ray temporal subtraction is described and claimed in pending application Ser. No. 358,741, filed Mar. 15, 1982. Hybrid subtraction methods and apparatus are disclosed and claimed in pending application Ser. No. 271,683, filed Apr. 26, 1982. Both of the aforementioned applications are assigned to the assignee of this application. The basic hybrid subtraction method and apparatus for performing the same are described in pending patent application Ser. No. 260,694, filed May 5, 1981 wherein W. R. Brody is the inventor.
Temporal subtraction is one well known procedure for enhancing visualization of blood vessels to the exclusion of surrounding soft tissue and bony structures. In temporal subtraction, an x-ray image of the blood vessels in a region of interest in the body is acquired just before an opaque x-ray contrast medium, such as an iodinated compound, that has been injected in the circulatory system arrives in the vessels of interest. This is called a pre-contrast mask image and it contains the vessels and usually a background of soft tissue and bony structures. The precontrast mask image is digitized and the digital data representative of the picture elements (pixels) in the image are placed in a digital frame memory. When the contrast medium reaches the vessels in the region of interest, a series of x-ray images are made and they are converted to digital data. The mask or pre-contrast image data are then subtracted from the post-contrast image data to cancel or subtract out all soft tissue and bony structure and anything that is common to both images to thereby enhance visualization of the blood vessels that contain the contrast medium. Usually the x-ray tube current and applied kilovoltages are the same for the pre-contrast and post-contrast images. The method is called temporal subtraction imaging because of the substantial time lapse between the pre-contrast and post-contrast images. As is known, the pre-contrast mask images and post-contrast images will always have some noise content that is introduced by the x-ray system and the electronic components that are used to generate and process the signals that represent the image.
Temporal subtraction provides high signal-to-noise ratio (SNR) and is a preferred procedure in cases where there is little if any movement of soft tissue during the interval between acquisition of the pre-contrast and post-contrast images. However, where there is tissue motion there must necessarily be information that is not common to successive images so this information does not cancel out by subtraction and contrast of the contrast mediumfilled vessel and the surrounding residual tissue is diminished. Blood vessel and surrounding tissue movement is likely to exist in abdominal vessel studies were peristalsis of the digestive organs moves the vessels. Renal artery studies are often adversely affected. Movement is also exhibited in carotid artery studies where the swallowing reflex causes an artifact which can obscure perception of the vessels of interest.
Another image subtraction procedure is characterized as energy subtraction. Energy subtraction is based on the fact that x-ray attenuation by a body or any material is an x-ray energy dependent phenomenon and that the energy dependence is different for materials having different atomic number averages. In energy subtraction, an x-ray image of a region of interest in the body is obtained with a nominally low kilovoltage (kV) applied to the x-ray tube so the beam projected through the body has an energy spectral distribution within a band having low average energy. After a low average energy image is obtained and digitized, at least one more image is obtained with a comparatively higher kV applied to the x-ray tube and a resulting higher average energy spectral band. For ordinary tissue studies the two images may be made in the absence of any contrast medium. For angiographic studies, the two images are obtained when there is an x-ray contrast medium such as an iodinated compound present in the blood vessels. In any case, the high average energy image pixel data are subtracted from the low average energy pixel data and a difference image remains. Prior to subtraction, the data are usually variously weighted or scaled to bring about cancellation of soft tissue. The data could be scaled to eliminate bone from the difference image instead of tissue. However, it is not possible to remove or cancel bony structures without also removing most of the iodinated contrast medium which is really what one is trying to visualize in angiographic studies since it defines the interior shape of the vessel.
There are also brightness monuniformities in the subtracted or difference images due to several effects when the data are acquired using an image intensifier that is coupled to a television camera. Veiling glare, which is like haze, results from light diffusion or scattering often present in the input or output phosphors of the image intensifier. The fact that rays of a broad x-ray beam are scattered by body tissue in an energy dependent manner between ray paths also causes loss of contrast in the difference image. Differential detection of x-rays at various energies in the input phosphor of the image intensifier leads to additional brightness nonuniformities. None of these phenomena can be completely nullified by energy subtraction alone.
Hybrid subtraction has been proposed for cancelling the contrast of stationary bone and soft tissue and elimination of artifacts due to soft tissue motion while still providing an image of the contrast medium filled vessels. Hybrid subtraction procedures use a combination of energy and temporal subtraction methods. In hybrid subtraction, x-ray images are obtained using two x-ray spectra having different average energies and are combined in a manner to suppress signals due to soft tissue in a heterogeneous object such as the body. Basically, in hybrid subtraction, a mask image is obtained first by projecting a low average energy x-ray beam (hereafter called low energy beam or low energy spectral band) through the body followed by a higher average energy x-ray beam (hereafter called high energy beam or high energy spectral band) when the injected x-ray contrast medium has not yet entered the blood vessels in the anatomical region of interest. The images, consisting primarily of bone and soft tissue acquired at two energies, are scaled and weighted using appropriate constants and then subtracted to produce a mask image in which signals due to soft tissue variations are suppressed or cancelled and bony structures remain. The data for a pair of high and low energy x-ray images are next obtained when the injected contrast medium reaches the vessels in the region of interest. The data for this pair of images are acted upon by the same constant weighting factors that are used with the first pair of images to cancel soft tissue. One image acquired in this exposure pair is subtracted from the other such that the resulting post-contrast image contains data representative of bone structures plus vessels containing contrast medium. The final step in hybrid subtraction is to subtract the dual energy post-contrast difference image from the dual energy pre-contrast mask difference image to thereby suppress or cancel the bone structures and isolate the contrast medium-containing vessels. A major advantage of hybrid subtraction over temporal subtraction alone is the reduced sensitivity to soft tissue motion artifacts because the soft tissue is suppressed or cancelled in the pre-contrast and post-contrast dual energy images.
Hybrid subtraction is superior for eliminating soft tissue structures that may have moved during the time between obtaining the mask image and post-contrast image or images. As indicated earlier, however, if there is no movement ordinary temporal subtraction is preferred because of its better SNR compared to hybrid subtraction.
Scattering of the x-ray beam by the body is also considered. Scatter in an image depends on x-ray beam energy, beam path length and the density of the object being penetrated. In hybrid subtraction the scattering that results from use of a wide area x-ray beam is of little consequence since scatter is essentially the same for each energy subtracted pair of images. Hence, scatter effects on image brightness nonuniformities are subtracted out when the dual energy difference image pairs are subtracted.
In hybrid subtraction as presently practiced, the multiple subtractions resulting from subtracting pre-contrast high and low x-ray energy images to obtain a first difference image and subtracting low and high x-ray energy post-contrast images to obtain a second energy subtracted difference image and then subtracting the two difference images results in a substantial reduction of the signal-to-noise ratio. To compensate for the decreased signal-to-noise ratio of hybrid subtraction imaging compared to temporal subtraction imaging, generally the x-ray tube current has to be increased or exposure times have to be increased or both. In such cases the time between the mid-points of the low and high pre-contrast and post-contrast x-ray exposures may have to become very long and this means that the energy subtracted images themselves may have some motion artifact which is the exact effect the hybrid subtraction method is supposed to suppress. It will be evident then that hybrid subtraction as presently constituted involves a balancing of efficient x-ray dose utilization, motion artifact reduction, and signal-to-noise ratio, none of which have been optimized until the invention disclosed herein was made.
Recursive filtering has been proposed for reducing the effect of noise in temporally subtracted x-ray images, that is, in the difference image that results from subtracting a mask image obtained at one time from a live contrast medium exhibiting image obtained shortly thereafter. Recursive filtering is discussed at length in previously cited pending application Ser. No. 358,741. Recursive filtering in temporal subtraction systems but not in hybrid subtraction systems was recently described in several articles: Kruger, R. A. "A Method for Time Domain Filtering Using Computerized Fluoroscopy": Medical Physics, Vol. 8, No. 4, July/August 1981, pp. 465-469; Kruger, R. et al, "Time Domain Filtering Using Computerized Fluoroscopy--Intravenous Angiography Applications", SPIE Vol. 314 Digital Radiography (1981), pp. 319-326; Gould, R. G. et al "Investigation of a Video Frame Averaging Digital Subtraction System," SPIE Vol. 314, pp. 184-190 (1981); and Gould, R. G. et al "A Digital Subtraction System with Tandem Video Processing Units," SPIE Vol. 273, pp. 125-132 (1981).